Has a lot to do with sound deadening. Furthermore, it can be argued that that liner has more durability than polyurethane or whatever polymer mix is in the old liners.
Mitsubishis have had these in some form or fashion for a while now. I know the Evos have em too, but mostly on the top portion of the wheel well. Downside is that if your wheels rub, they do tend to tatter easier than smooth plastic :/